1. Field of Invention
The present invention relates in general to an equalization technique, and more particularly to an equalization technique for a channel with multiple clusters.
2. Related Art
In a wireless communication environment, there is a multi-path phenomenon due to diffractions and refractions of electromagnetic waves caused by obstacles. Therefore, when a channel thereof is observed via a time domain perspective, the channel may have a plurality of delay paths. Moreover, when the channel is observed via a frequency perspective, the channel may be regarded as a frequency-selective channel.
Taking a present code division multiple access (CDMA) system as an example, to solve the problem of interference from the frequency-selective channel, a receiver of the CDMA system generally applies an equalization technique for equalizing the frequency-selective channel. In other words, an equalizer is used for equalizing the frequency-selective channel to be a frequency-flat channel, so as to reduce the multipath interference in the received signals.
FIG. 1 is a system block diagram of a receiver of a conventional CDMA applying an equalizer. Referring to FIG. 1, a channel response of a received signals r[m] estimated by a channel estimation unit 110, namely, a delay time τ of each delay path within the channel and a channel gain ĥ(τ) corresponding to each delay time are estimated, and a plurality of weights w0, w1, w2, . . . , wF-1 of an equalizer 130 are calculated according to the estimated channel gains ĥ(τ), and then the weights w0, w1, w2, . . . , wF-1 are output to the equalizer 130. Next, the equalizer 130 sequentially delays the received signals r[m] for a chip duration TC. Then, after respectively multiplying the original received signal r[m] and the delayed received signals r[m−1], r[m−2], r[m−F+1] with the weights w0, w1, w2, . . . , wF-1, a sum of above multiple multiplications is then outputted by the equalizer 130. A correlator 150 de-spreads the equalized signal processed by the equalizer 130 according to a spreading code c[n] of a client, and then a decision unit 170 demodulates a digital signal {circumflex over (b)}.
A window length of the equalizer 130 is represented by F. For a present equalization technique, a plurality of documents (for example, note [1]) refers to that the window length F of the equalizer has to be greater than or equal to double of the channel length thereof, so that the equalizer may effectively eliminate the multipath interference to the received signals. Therefore, as to the hardware of the receiver, if the window length of channel estimation is L, the window length F of the equalizer is then designed to be 2 L.
However, in case of a relatively serious channel delay spread, the length of an actual transmission channel is greatly increased, as shown in FIG. 2. FIG. 2 is a diagram illustrating a channel power delay profile. Referring to FIG. 2, horizontal axis thereof represents delay times, and vertical axis represents the channel power on its corresponding delay time. According to FIG. 2, the delay paths within the channel is sparsely distributed on time domain, and the delay paths may be grouped into several clusters of cluster 1, cluster 2 . . . cluster P. A reason of such channel phenomenon may be that in a hilly terrain (HT), the electromagnetic waves emitted from the transmitter are received by the receiver after a long distance reflection, so that the delay paths of the different clusters are generated. Alternatively in the case of soft handover (SHO), the receiver may be just located within transmission ranges of multiple base stations, so that the receiver may simultaneously receive signals from the multiple base stations, and therefore the delay paths of the multiple clusters are generated.
Due to a limitation of the hardware, if the window length of the equalizer of the receiver maintains to be F=2 L, the window length of the equalizer may not be enough to cover all the delay paths, so that the equalizer cannot effectively equalize the transmission channel, and accordingly performance of the receiver is degraded.
A U.S patent publication No. 2006/0109892 A1 provides a receiver having two equalizers, as shown in FIG. 3. The two equalizers 335 and 340 of the receiver 300 equalize the received signals respectively based on delay paths 305A and 305B of two clusters. Next, the signals equalized by the two equalizers 335 and 340 are combined for outputting to a CMIS circuit 352. The CMIS circuit 352 regenerates the signal and feeds back the regenerated signal to adders 325 and 330 for multipath interference cancellation.
When the weights are calculated, calculation of a weight of the equalizer 335 only considers a channel response of the delay path 305A from the first cluster, and calculation of a weight of the equalizer 340 only considers a channel response of the delay path 305B from the second cluster. In other words, the weights of the equalizers 335 and 340 are not calculated under a minimum mean square error (MMSE) criterion. Actually, when a signal is transmitted within the channel, the signal received by the equalizer 335 is interfered by the delay path 305B of the second cluster; however, the equalizer 335 only takes into account the delay path 305A of the first cluster. Similarly, the signal received by the equalizer 340 is interfered by the delay path 305A of the first cluster; however, the equalizer 340 only takes into account the delay path 305B of the second cluster. Therefore, though the two equalizers 335 and 340 are applied in the aforementioned patent, interferences of the delay paths 305A and 305B cannot be simultaneously mitigated. Since the equalizers 334 and 340 cannot totally eliminate the interferences within the channel, the signal restored by the CMIS circuit still has the interference. However, the restored signal with the interference is still fed back to the adders 325 and 330, so that an error propagation phenomenon occurs. Moreover, when signal energy received by the receiver is relatively small, such feed-back mechanism may lead to an excessive small signal-to-interference plus noise ratio (SINR) of the receiver, and accordingly the performance of the receiver is degraded.
A receiver with multiple equalizers is provided in US Publication No. 2003/0133424 A1 as shown in FIG. 4. FIG. 4 is a system block diagram illustrating a receiver published in US Publication No. 2003/0133424 A1. The receiver 400 includes a plurality of equalizers 408A—408C for receiving the received signals from a plurality of antennas and equalizing the received signal respectively. After that, the equalized signals equalized by the equalizers 408A˜408C are operated by time-alignment and dispreading, the combiner 311 combines the dispreading signals to recover the original signal.
When the weights of the equalizer 408A˜408C are calculated, the calculation of the weights utilizes a method of direct matrix inversion under a minimum mean square error (MMSE) criterion. Actually, due to the calculation of direct matrix inversion, the arithmetic complexity of the receiver 400 is greatly increased. Also, considering the implementation of hardware, the hardware complexity of the receiver 400 should be limited so that the window length of the equalizers 408A˜408C must be limited. Therefore, the equalizers 408A˜408C may not be able to eliminate the interference of the received signals while the received signals are transmitted by the longer length of the transmission channel.
Note [1]: M. Melvasalo, P. Jänis and V. Koivunen. “Low complexity space-time MMSE equalization in WCDMA systems,” proc. of 2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications, Berlin, Germany, pp. 306-310, 2005.